Check your results by inspecting the XML file.
Do you see any RX packet? Or is it zero?
The python script will try to calculate the rxBitrate as follows:
if rx_duration > 0:
self.rxBitrate = long(flow_el.get('rxBytes'))*8 / rx_duration
else:
self.rxBitrate = None
If you do not receive any packet, it will be assigned None which does not print as floating
print "\tRX bitrate: %.2f kbit/s" % (flow.rxBitrate*1e-3,)
This is a bug in the script and should be fixed (I will report it).
But in terms of your results, it tells you that (a) you should read the error message and investigate yourself and (b) check your scenario as you do not have any received packet.